1. Field of The Invention
The present invention relates to a method of packaging wafers, and more particularly, to a method of packaging wafers to prevent the wafers from being contaminated by the external environment.
2. Description of the Related Art
The wafers used in manufacturing semiconductor devices are mainly comprised of silicon crystal. In order to achieve more precise device fabrication and greater device yields, contamination free wafers are essential, and appropriate packaging is required to prevent such contamination from the external environment. Bare wafers, as well as those having semiconductor devices formed thereon, must sometimes be stored for a predetermined period, and therefore should be packaged and stored properly to avoid contamination.
Packaging bags made of a laminated film have been employed to store the wafers. The laminated film generally consists of a polyethylene film, an adhesive film, and an aluminum-coated polyethylene film that are stacked successively. In order to prevent contaminating particles from the external environment from contacting the wafer or the packaging bag, several methods of packaging have been employed. These methods include: flushing the package with nitrogen and then sealing (nitrogen flushing package); reducing the vacuum pressure of the package for more than 4.5 seconds and then sealing (vacuum reduced pressure package); and placing a wafer in a reinforced packaging bag and then reducing the vacuum pressure of the package for about 3 seconds and then sealing (reinforced packaging bag package). The reinforced packaging bag includes an additional polyethylene film layer and an additional aluminum-coated polyethylene film layer that are added to the structure of the prior laminated bag.
While the nitrogen flushing package and vacuum reduced pressure package both are theoretically designed to eliminate contaminating particles in the packaging bag into which the wafer is put, it has been found that such packaging methods tend to increase the contaminating particles on the wafer, thereby decreasing the storage stability of the wafer.
With regard to the wafer contamination, if sulphuric oxide (SO.sub.x) is adsorbed onto the surface of the wafer, it creates a haze that contaminates the surface, and acts as a nucleation source for further particulate contamination. As shown in the graph of FIG. 1, the amount of sulphuric oxide on the respective surfaces of several wafers increases from about 2.3.times.10.sup.12 atoms/cm.sup.2 after one hour of exposure to the external air, to about 3.3.times.10.sup.12 atoms/cm.sup.2 after one day of exposure, and to about 4.5-6.0.times.10.sup.12 atoms/cm.sup.2 after three days of exposure. Thus, the amount of time the wafer is exposed to the external air prior to packaging is an important factor affecting the initial sulphuric oxide contamination concentration. The sulphuric oxide contamination measurements for the example above, as well as those described below, are performed using a Total Reflective X-Ray Fluorescent Spectroscopy instrument (TRXRF), manufactured by Rikaku Co. of Japan. Also, note that the various symbols in the figures refer to different wafers, although some of the symbols in the graphs overlap.
FIGS. 2-7 are graphs depicting various concentration amounts of sulphuric oxide contamination on a wafer exposed to the external air containing sulphuric oxide for time periods of one hour, one day, and three days. The wafers are then packaged according to the conventional packaging methods as described above, and thereafter stored for predetermined time periods, namely 23 days and 45 days. FIGS. 2, 3 and 4 depict the amount of sulphuric oxide contamination on a wafer that was exposed to external air containing sulphuric oxide for time periods of one hour, one day, and three days, and then stored for 23 days in a conventional nitrogen flushing package (in FIG. 2), vacuum reduced pressure package (in FIG. 3), and reinforced packaging bag package (in FIG. 4). FIGS. 5, 6 and 7 depict the amount of sulphuric oxide contamination on a wafer that was exposed to external air containing sulphuric oxide for time periods of one hour, one day, and three days, and then stored for 45 days in a conventional nitrogen flushing package (in FIG. 5), vacuum reduced pressure package (in FIG. 6), and reinforced packaging bag package (in FIG. 7).
As shown in FIGS. 1-7, the relatively small fluctuations in sulphuric oxide on the wafer surface indicate that the packaging types and period of storage have little effect on the initial contamination concentration of sulphuric oxide. However, while the initial contamination concentration of sulphuric oxide acts only as the haze's nucleation source, the longer the period of storage the greater is the acceleration of the nucleation, and the greater is the probability of particulate contamination. Although one of the "three day exposure" data points for the contamination concentration in FIG. 7 appears low, this is considered to be an experimental and/or measurement error.
FIGS. 8-13 show the increase or decrease in the number of contaminating particles under the conditions identified in FIGS. 2-7, as classified by particle size. More specifically, FIGS. 8, 9 and 10 depict the particle size distribution on a wafer that was exposed to external air containing sulphuric oxide for time periods of one hour, one day, and three days, and then stored for 23 days in a conventional nitrogen flushing package (in FIG. 8), vacuum reduced pressure package (in FIG. 9), and reinforced packaging bag package (in FIG. 10). FIGS. 11, 12 and 13 depict the particle size distribution on a wafer that was exposed to external air containing sulphuric oxide for time periods of one hour, one day, and three days, and then stored for 45 days in a conventional nitrogen flushing package (in FIG. 11), vacuum reduced pressure package (in FIG. 12), and reinforced packaging bag package (in FIG. 13). A Surfscan-6200 measurement apparatus (manufactured by KLA & TENCOR Co.), was used to ascertain the particle measurements.
For the nitrogen flushing packages of FIGS. 8 and 11, there is shown a rapid increase in particles on a wafer exposed for three days and stored for 23 days (FIG. 8), and exposed for both one day and three days and stored for 45 days (FIG. 11).
The increase or decrease in the particles shows no definite tendency for the vacuum reduced packages of FIGS. 9 and 12. Such irregular results make it difficult to predict the storage stability of the wafer.
For the reinforced packaging bag packages shown in FIGS. 10 and 13, the particulate contamination increases significantly on a wafer exposed for three days and stored for 23 days and 45 days.
From the above results, several possibilities arise as causes for the deterioration of wafer storage stability. For example, in the case of the nitrogen flushing method, the nitrogen supply includes a filtering means installed on a nitrogen supply line for filtering foreign matter contained in the nitrogen, such as particulates. The particulates that accumulate in the filtering means are emitted at irregular and unpredictable intervals during the nitrogen flushing, which accumulate in the packaging bag. Therefore, frequent cleaning of the nitrogen supply line or frequent filter changes are required, which increases production costs while reducing productivity.
In case of the vacuum reduced pressure method, the air in the packaging bag is drawn out and expands rapidly during the method, which results in a large temperature decrease on the wafer surface, thereby causing the sulphuric oxide in the air to condense and adhere to the wafer surface and increasing the particulate contamination.
In the reinforced packaging bag method, the particulate contamination is lower than the vacuum reduced pressure package because the vacuum application time is shorter (less than 3 seconds) than in the normal vacuum reduced pressure package (more than 4.5 seconds).
Accordingly, a need exists for a wafer packaging method in which a wafer, or a semiconductor device formed on the wafer, can be stored for a long period of time in a stable manner.